The manufacture of hydraulic binders, and cements in particular, consists essentially in a calcination of a mixture of raw materials which have been judiciously selected and proportioned, also designated by the term “raw mix”. Baking this raw mix gives an intermediate product, a clinker, which, when milled with possible mineral additions, produces cement. The type of cement produced depends on the nature and proportions of raw material as well as the baking method. There are distinguished several types of cements: Portland cements (which represent the greatest majority of cements produced in the world), aluminate cements (or calcium aluminate), natural quick-setting cements, sulfo-aluminate cements, sulfo-belite cements and other intermediate varieties. As these families are not entirely separate, it is preferable to describe them by their chemical and mineralogical constituents.
The most widespread cements are Portland cements. Portland cements are obtained from Portland clinker, obtained after clinkering at a temperature of the order of 1450° C. of a raw mix rich in calcium carbonate in a furnace.
The drawback of preparing such cements is that they release a lot of CO2. The cement industry is hence today on the lookout for an alternative equivalent to Portland cement, that is to say, having at least the same resistance and quality features as Portland cements, but which, during their production, release less CO2.
In this respect, these last few years, research has been oriented towards cements called sulfo-aluminate and sulfo-belite, which, during their production release less CO2 than Portland cements.
The clinker being the result of a calcination at high temperature, the elements are essentially present in the form of oxides. Clinkers allowing the preparation of sulfo-aluminate cements or sulfo-belite cements relate to a method of producing a clinker from a raw mix constituted by a mixture comprising CaCO3, Al2O3, and/or Al(OH)3 CaSO4, SiO2, Fe2O3 compounds and/or a product containing silica or silicates such as clay, all these compounds being present in anhydrous or hydrated form, individually or in combination. The raw mix may be prepared with any natural or synthetic mineral materials able to supply calcium, silicon, sulfur, iron and aluminum.
As part of this research, numerous sulfo-aluminate clinkers have been described. It can be cited for example international patent application WO-A-2006/018569 describing sulfo-aluminate belite clinkers comprising 5 to 25% calcium aluminoferrite phase of a composition corresponding to general formula C2AF(1-x), with x comprised between 0.2 et 0.8; 15 to 35% of a calcium sulphoaluminate “yee' limit” phase (C4A3$); 40 to 75% belite (C2S); and 0.01 to 10% of one or several minor phases. As mentioned in this patent application, such clinkers contain, in comparison with the alite phase (C3S), the main component of Portland cements, a higher quantity of belite phase (C2S), which is entirely beneficial, since it leads to reducing industrial emissions of CO2 and energy consumption. However, belite contributes to the development of the long term resistance of belite sulfo-aluminate cement. Nevertheless, most clinkers described in this patent application contain boron, thus being an economic drawback as regards cost and rarity of this constituent.
As to the international patent application WO-A-2012/010800 it describes iron-doped sulfo-belite clinkers comprising from 5 to 60% iron-doped calcium sulfoaluminate phase corresponding to formula C4AxFy$z with x varying from 2 to 3, y varying from 0 to 0.5 and y being different from 0, and z varying from 0.8 to 1.2; from 0 to 25% calcium aluminoferrite phase of a composition corresponding to general formula C6Ax′Fy′, with x′ varying from 0 to 1.5 and y′ varying from 0.5 to 3; from 20 to 70% belite C2S phase; and less than 10% C11S4B phase. Iron doping is described as allowing the preparation of cements having a hydraulic reactivity and an increased resistance in comparison with clinkers described in the international patent application WO 2006/018569, while allowing to reduce CO2 emissions by almost 35% during their preparation in comparison with Portland type clinkers. In addition, these clinkers do not require the presence of boron nor the addition of additives for improving the quality of the prepared cements and concretes.
The setting time of the hydraulic binders such as cement, and more generally of grout, concrete and mortar prepared from these hydraulic binders, starts from the mixture and kneading when manufacturing them. The transportation hence initiates this setting time and must be as rapid as possible in order to preserve maximum maneuverability of the grout, concrete and mortar during their placing. By way of example, the average time period for the transportation and implementation of the concrete is two hours, beyond this time period, the setting of the concrete has already started and the quality thereof is no longer ensured.
Thus, despite the setting time of the hydraulic binders prepared from clinkers described in the prior art being compliant with the applicable norms, it still remains interesting for those skilled in the art to increase said setting time as much as possible while maintaining the hydraulic reactivity and the resistance of the prepared materials in the short, medium and long term.
It therefore remains interesting to identify new clinkers able to be prepared at temperatures largely lower than 1425° C., thus strongly reducing CO2 emissions during their preparation in comparison with “Portland” clinkers, which allow obtaining hydraulic binders that benefit from an improved setting time while maintaining their hydraulic reactivity and their resistance in the short, medium and long term.
In the past, numerous publications have pointed out the existence of correlation between a rapid setting time and the presence of the C4A3$ phase in the clinker. Thus, an article called “Understanding expansion in calcium sulfoaluminate-belite cements”, Cement and Concrete Research 42 (2012), 51-60, Chen et al. explain that sulfoaluminate-belite cements have shown a rapid setting time due to the reactivity of the C4A3$ phase. Similarly, “Characteristics of cement pastes containing sulphoaluminate and belite prepared from nano-materials”, Construction and building materials 38 (2013) 14-21, El-Didamony et al. state that cements containing anhydrite have a shorter setting time and a better resistance in short deadlines.
However, it has now been found in an entirely surprising manner that the belite depletion of certain sulfo-aluminate clinkers allowed to significantly increase the setting time of the hydraulic binders prepared from these clinkers while having a hydraulic reactivity and a resistance comparable to or higher than those of cements prepared from the clinkers described in the international patent application WO-A-2012/010800, and allowing to reduce CO2 emissions by almost 35% during their preparation in comparison with Portland type clinkers.